Stormwater runoff is water generated by rain contacting impervious and semi-pervious land surfaces which contains significant amounts of contaminants. These contaminants are routinely transported to and discharged into downgradient wetlands, streams, lakes and coastal waters (EPA 1983).
Stormwater pollutants include bacteria, viruses, metals, nutrients, oils and other organic compounds. Throughout the United States, stormwater pollution has resulted in the closure of shellfish beds, the eutrophication of waters, the pollution of drinking water supplies and impacts to ecological habitats.
Numerous studies performed by have demonstrated that the first flush of runoff contains the majority of the pollutants. This is because many of the contaminants are associated with sediment particles which accumulate on road and parking lot surfaces between storms and are transported during the beginning of the first significant precipitation event. Contaminants such as oils and other hydrocarbons also collect on impervious surfaces and are typically "flushed" during the beginning of a storm event.